Question

The reaction \(\mathrm{C}_{6} \mathrm{II}_{6}+\mathrm{Br}_{2}\) \mathrm{ Febr, } \({\longrightarrow} \mathrm{C}_{6} \mathrm{II}_{5} \mathrm{Br}+\mathrm{IIBr}\) is (1) an electrophilic addition reaction (2) a nucleophilic substitution reaction (3) an electrophilic substitution reaction (4) a free radical substitution reaction

Short answer

This is an electrophilic substitution reaction (Option 3).

Step-by-step solution

Identify the type of reactants and products involved in the reaction

Observe the reactants and products given in the reaction. The reactants are \(\text{C}_{6} \text{H}_{6}\) and \(\text{Br}_{2}\), and the products are \(\text{C}_{6} \text{H}_{5} \text{Br}\) and \(\text{HBr}\).

Understanding the electrophile and nucleophile

In this reaction, \(\text{Br}_{2}\) acts as the electrophile (electron-seeking species) because it reacts with \(\text{C}_{6} \text{H}_{6}\) (benzene), causing a substitution. Benzene typically undergoes electrophilic substitution due to the high electron density of the aromatic ring.

Analyze the reaction mechanism

Benzene \(\text{C}_{6} \text{H}_{6}\) undergoes electrophilic substitution when \(\text{Br}_{2}\) in the presence of a catalyst (FeBr3) reacts with it. This leads to the formation of bromobenzene \(\text{C}_{6} \text{H}_{5} \text{Br}\) and HBr.

Determine the correct type of reaction

Based on the analysis, it is clear that benzene undergoing substitution by \(\text{Br}_{2}\) to form bromobenzene and \(\text{HBr}\) is an electrophilic substitution reaction.

Verify your answer with the given options

Among the provided options, the reaction fits the description of 'an electrophilic substitution reaction.'

Key concepts

Electrophiles and Nucleophiles

One of the key concepts to grasp in organic chemistry is the difference between electrophiles and nucleophiles. Electrophiles are species that seek electrons. They are attracted to areas of high electron density and are often positively charged or electron-deficient. Examples of electrophiles include \text{H}\(^{+}\), \text{Br}\(^{+}\), and \text{NO\(_2^{+}\)}.

Nucleophiles, on the other hand, are species that donate electrons. They are attracted to positive charges and are usually negatively charged or possess lone pairs of electrons. Examples of nucleophiles include \text{OH}\(^{-}\), \text{CN}\(^{-}\), and amines.

In the reaction we are examining, \text{Br\(_2\)} serves as the electrophile because it seeks out the electron-rich benzene ring to add to. Benzene itself, with its high electron density, acts as the nucleophile.

Aromatic Compounds

Aromatic compounds are a special class of molecules characterized by a ring structure with alternating single and double bonds, known as conjugation. Benzene (\text{C}\(\text{H}_6\)) is the most well-known example. The structure of benzene consists of six carbon atoms connected in a ring, with each carbon also bonded to one hydrogen atom.

These alternating bonds create a system of delocalized \text{pi} electrons above and below the plane of the ring, providing chemical stability and unique reactivity to aromatic compounds. This high electron density makes benzene particularly reactive towards electrophiles.

The reaction involving the substitution of a bromine atom onto a benzene ring is an example of how aromatic compounds undergo electrophilic substitution rather than addition, preserving their stable ring structure.

Reaction Mechanisms

Understanding the reaction mechanism is crucial to predict the products of a chemical reaction. The reaction mechanism explains the step-by-step process through which reactants transform into products.

In the electrophilic substitution reaction involving benzene, the mechanism proceeds as follows:

• Step 1: Activation of the Electrophile — \text{Br\(_2\)} reacts with the catalyst (FeBr\(_3\)) to form a more reactive bromonium ion (\text{Br}\(^+\)).


• Step 2: Formation of the Arenium Ion — The electron-rich benzene ring donates \text{pi} electrons to the \text{Br}\(^+\) ion, forming a highly reactive intermediate known as an arenium ion.


• Step 3: Restoration of Aromaticity — The arenium ion loses a \text{H}\(^+\) ion (proton) to regain the stable aromatic structure of benzene, resulting in the formation of bromobenzene (\text{C\(_6\)H\(_5\)Br}) and hydrochloric acid (\text{HBr}).

Each of these steps involves precise electron movements and temporary formations of high-energy intermediates, making the understanding of reaction mechanisms vital for mastering organic chemistry.